Embodiments of the present invention relate to gain guiding and index antiguiding of laser radiation in a waveguide, and more particularly, to apparatuses and method for performing gain guiding and index antiguiding of laser radiation in a microstructured waveguide.
High-power waveguide lasers and amplifiers can enable a broad spectrum of military and civilian applications. These include Light Detection And Ranging (LIDAR) for ranging, tracking and target identification; obstacle avoidance systems for unmanned vehicles; improved free-space laser communications (ground-to-air, air-to-air, and inter-satellite); coherent laser radar for wind metrology and vibrometry; pump sources for nonlinear frequency down-conversion for counter-measures; clear-air turbulence analysis; bio-chemical detection and pollution monitoring; and high power laser weaponry. Among various platforms of high-power sources, fiber lasers are particularly attractive due to their light weight, high conversion efficiency, ease of thermal management, and near diffraction-limit beam quality. In addition, optical fiber material is compatible with a variety of doped ions, offering a wide range of choice of photon energy depending upon a specific application.
Conventional waveguides and fibers comprise a core surrounded by a cladding. The core has an index of refraction higher than that of the cladding, i.e., a positive index step, such that light can be trapped inside the core by total internal reflection. This is referred to as “index guiding.” The shape, dimension and the index step determine the modal characteristics of guided modes, including field distribution, polarization, and dispersion.
In many applications, single mode operation is highly desired. The fundamental mode in fibers has the least diffraction in free space to maintain small focus over a longer distance. Its simple field pattern facilitates coherent beam combining of multiple apertures. However, to achieve single-mode operation requires a combination of small index step and/or a small core diameter, which limits the modal area and therefore output power of the fiber laser. For example, conventional step-index fiber has a maximum diameter of only around 15 μm in order to remain single-mode.
To maximize the obtainable power, both high output power from a single aperture and coherent beam combining of multiple apertures are desirable. Power scaling of single fiber lasers is limited by optical damage of the host materials and optical nonlinearity. The latter includes stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and self-phase modulation (SPM), all of which convert laser radiation to other photon energy. These linear and non-linear impairments depend on laser intensity, which call for novel fiber design with large modal area (LMA) to reduce peak intensity while attaining high output power. Single-mode operation and LMA are also critical for coherent beam combining since the former allows for simple and predictable interference pattern at the far field and the latter suppress SBS and SRS, which impose noise-like wide band modulation on the amplified beam to degrade constructive interference.
Power threshold of stimulated Brillouin scattering and stimulated Raman scattering is inversely proportional to the fiber length. Higher threshold of these impairments calls for a shorter device length. To obtain high power over a short distance requires a large spatial overlap between the pump and laser radiation to provide a high gain. See, e.g., Limpert et al., “High-power rod-type photonic crystal fiber laser,” Optics Express 13(4), 2005, pp. 1055-1058.
Efficient coupling of pump radiation into fibers is also required to obtain high power. Conventionally this is done by “cladding-pumped,” meaning that the fiber has a core surrounded by an inner cladding layer, which is then surrounded by an outer cladding layer. FIGS. 1 and 2 are representative of a cladding-pumped fiber, showing the fiber 10 having a core 12 surrounded by an inner cladding layer 14 that is then surrounded by an outer cladding layer 16. The core 12 and inner and outer cladding 14, 16 have respective indices of refraction nc, nic, noc and are chosen such that nc>nic>noc. This causes signal or laser radiation propagating through the core 12 to be confined in the core, and pump radiation to be confined by the inner cladding 14 via total internal reflection. In particular, the inner cladding can be much larger than the core in size and have much larger refractive index than the outer cladding. Both large size and large numerical aperture substantially facilitates pump coupling.
A single-mode LMA fiber laser based upon gain guiding in a large-core index-antiguided fiber is the subject of the disclosure in U.S. Pat. No. 6,751,388 to Siegman (“the '388 patent”). In index antiguided fibers, the core has a refractive index lower than the cladding such that all propagating modes therein are leaky. The lowest-order mode (i.e., the fundamental mode) has the lowest propagating loss, with increasing propagation loss for increasing higher-order modes. A suitable gain is provided by optical excitation of the dopant in the core to amplify laser radiation such that only the fundamental mode has net gain and becomes amplified while all other higher-order modes remain lossy and therefore decay during propagation. Theory shows that the threshold of gain guiding of the first higher-order mode in a step-index index antiguided (“IAG”) fiber is 2.54 times higher than that of the fundamental mode, which is very promising for strong transverse mode discrimination during gain guiding. Large IAG is shown critical to the realization of gain guiding (“GG”) by substantially reducing the threshold for gain guiding, thus making GG more practical in terms of fabrication and pumping requirements. General teachings on gain guiding in step-index fibers can be found in Siegman, “Propagating modes in gain-guided optical fibers,” Journal of the Optical Society of America A, Vol. 20, 2003, pp. 1617-1628 and “Gain-guided, index-antiguided fiber lasers,” Journal of the Optical Society of America B, Vol. 24, 2007, pp. 1677-1682.
Lasing in gain-guided index-antiguided fibers has been demonstrated in a flash lamp-pumped Nd+-doped phosphate fiber with core diameter up to 400 μm. From these experiments it was found that larger index antiguide is desired to reduce the gain to realize gain guiding. It has subsequently been found that index antiguiding also takes place for pump radiation, and unlike laser radiation where gain can compensate for its loss and confine its propagation, there is no gain for pump radiation which remains leaky and diffracts all its power during propagation.
Photonic crystal fibers are fibers in which the core is surrounded by photonic crystal structures, which may include, for example, a periodic array of holes having spacing on the order of light wavelength. A photonic crystal structure has a range of frequencies and propagation constants, known as “photonic bandgap,” over which propagation modes cannot be supported. Light propagates inside the core that falls within the photonic bandgap of the cladding will be trapped inside the core to form a confined mode. Single-mode operation can be obtained by proper choice of the frequency, the structure of the core, and the structure of photonic crystal cladding. Since this guidance mechanism does not rely on index guiding, light can be guided in a core where its index is lower than the average refractive index of the surrounding cladding.
The use of hollow-core photonic crystal fibers as high-power fiber laser sources can be found in U.S. Pat. Publ. No. 2005/0105867 to Koch et al. Laser radiation resides in the stopband of a photonic crystal cladding adjacent to the core, and pump radiation resides in the stopband of another photonic crystal cladding surrounding the inner photonic crystal cladding. Suppression of SBS is mainly achieved by the hollow core where a large fraction of mode energy is present. The method, however, yields small mode size since it is determined by the photonic bandgap guidance. Furthermore, amplification of the fundamental mode only takes place in the peripheral of the mode where the pump and laser radiation overlap and is not very efficient.
A fiber laser having a large core diameter that propagates only the fundamental mode with high gain to produce high power output has not yet been solved in the art. There remains a need in the art for a fiber laser with a large core diameter that only operates in fundamental mode and possesses enough gain to yield high power output.